Its theoretical, or worse case scenario what can happen. The cops DO know when your on a restricted licence, it comes up on the DVLA database immediately when they check your licence.
As for power increase, well if you advertise that your bike is 'tuned' such as end can, k&N stickers etc, then they don't need to prove iimmediately that it increases power, but they can sieze your bike anyway as I previously explained.
A beat bobby might not have a clue, but a traffic cop can do this.
As for fitting a filter not improving the power, well the restrictor kit is designed with a standard filter fitted, and then you put a less restricted filter in, it 'could' slightly increase the power. As I said, its just a theory.
And the cops don't always physically inspect a machine after a prang, they mostly do it at a serious or fatal, or where there are other offences to be proved. (such as power upgrades)